This invention relates to radio frequency modulation and in particular to a radio frequency modem having a receiver in which the DC bias in two unbalanced signal paths cancel out.
A voltage reference for automatic gain control is often achieved by using a voltage divider network of resistors tied between a supply voltage and ground. The voltage reference for automatic gain control thus generated does not float and relies on both the regulation of the power supply and the bias stability of the receiver's amplifiers and detectors, to prevent drifting of the detected modulation due to external effects such as changes in the temperature of components.
The receiver described below is used to demodulate a duobinary waveform in which digital, or binary, data has been encoded into and are represented by several levels of an amplitude modulated (AM) carrier for purposes of radio frequency (RF) stability (i.e. preventing unwanted oscillations) and also low component count and thus cost, it is necessary to keep overall receiver gain low. This means that the detected envelope of the signal will be small, on the order of three to four hundred millivolts. A modem must accurately divide, or slice, the detected envelope, or video signal, into four levels which must be, for the purpose of maintaining low bit error rate precisely defined over a wide operating temperature range and with varying component tolerances.
A single silicon diode forward voltage drop, such as a transistor base-to-emitter junction, is typically approximately 0.7 volts at room temperature. A forward voltage drop will typically vary with temperature two millivolts per degree centigrade. At this rate over a 100 degree centigrade operating range, a 200 millivolt variation would occur in a diode forward voltage drop which would be unacceptable when attempting to split a 300 millivolt video signal into four equal 75 millivolt windows in a circuit employing silicon devices. Some form of compensation for the temperature dependent forward voltage drop of a silicon diode must be utilized.
To cancel out the effects of temperature dependent DC bias introduced by the silicon diode forward voltage drop, many radio frequency amplifiers employ balanced amplifiers at each stage of amplification in order to prevent DC bias levels from drifting with temperature. While balanced amplifiers can be used to overcome the temperature dependent characteristics of silicon devices, a trade off is made in that circuits become much more complex due to the increased number of components.
It would be desirable to have a technique to overcome the undesirable effects of temperature variation on a forward voltage drop of silicon diodes that would not necessitate the use of balanced amplifiers at each stage of amplification.